Method for annealing silicon steel strip for use as material of electric machinery

ABSTRACT

Method for annealing silicon steel strip which has been coated with a slurry of such metal-hydroxides as are capable of forming a glassy film on the surface of said silicon steel strip, which has been wound into a coil, and which has been annealed in the desired atmosphere, thereby contracting the volume of the so applied metal-hydroxides, so as to produce a space between coillayers.

United States Patent Urushiyama et al.

[ 51 3,653,984 51 Apr. 4, 1972 [54] METHOD FOR ANNEALING SILICON STEEL STRIP FOR USE AS MATERIAL OF ELECTRIC MACHINERY [72] Inventors: Nobuo Urushiyama; Minoru Yosho; Takamitu Miyata, all of Kitakyushu City,

Japan [73] Assignee: Nippon Steel Corporation, Tokyo, Japan [22] Filed: Apr. 28, 1969 [21] Appl.No.: 819,876

2,533,351 12/1950 Carpenter ..l48/3l.5 X

3,037,889 6/1962 Nystrom ....148/1 13 X 3,084,081 4/1963 Carpenter..... ....l48/l 13 X 3,331,713 7/1967 Miller ....148/ll3 3,132,056 5/1964 McQuade 148/3 1 .5

FOREIGN PATENTS OR APPLICATIONS 892,652 3/ 1962 Great Britain 148/131 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. White Att0meyWenderoth, Lind & Ponack [57] ABSTRACT Method for annealing silicon steel strip which has been coated with a slurry of such metal-hydroxides as are capable of forming a glassy film on the surface of said silicon steel strip, which has been wound into a coil, and which has been annealed in the desired atmosphere, thereby contracting the volume of the so applied metal-hydroxides, so as to produce a space between coil-layers.

9 Claims, 6 Drawing Figures Patented April 4, 1972 FIG.

FIG. 4

FIG. 5A

wm a 3m 5 QRM UOmM wmA G NMT F INVENTOR 141M, ATTORNEY 5 METHOD FOR ANNEALING SILICON STEEL STRIP FOR USE AS MATERIAL OF ELECTRIC MACHINERY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates, in general, to a method for annealing silicon steel strip, and more particularly, to a method for annealing silicon steel strip used for making electric machinery, which strip is wound into a coil, in a box or a batch-type furnace.

2. Description of the Prior Art Silicon steel strip is generally wound into a coil so as to be subjected to annealing for such purposes as decarburizing and finishing-annealing. In general, however, such a coil is so tightly wound that an effective atmospheric gas cannot enter between coil-layers, making necessary a longer time for annealing. In order to solve this problem, there have been developed so-called loose coil type methods, according to which a space is produced between coil-layers, so that an atmospheric gas of high temperature can be driven through the space between coil-layers. For forming such loose coil there are available such methods as the one according to which metal wire is interposed between coil-layers and taken out just before the coil is charged into the annealing furnace, whereby there is formed a loose coil; the one according to which a coil is made with such metal wire or any other material that will produce a space between coil-layers, interposed between coillayers, and the thus formed loose coil is charged into the annealing furnace with said space-producing material; and the one according to which a coil is made with such combustible material as paper tape, interposed between coil-layers, and said combustible material is burned in the annealing furnace to produce a space between coil-layers, whereby there is formed a loose coil.

However, in order to make a loose coil by such methods, there must be used some space-producing materials, which is a disadvantage of such methods. Particularly, the above method wherein metal wire interposed between coil-layers is taken out just before the coil is charged into the annealing furnace, causes difficulty in handling the thus made coil because it must be transported uprightly and both ends of the coilheld fixedly, so as not to deform it.

SUMMARY OF THE INVENTION An object of the present invention is to provide a silicon steel strip annealing method developed to overcome the drawbacks of the conventional methods in which a loose coil is formed very easily and efficiently, and concurrently a film which is also useful as insulation, is formed on the surface of silicon steel strip.

Another object of the present invention is to provide a method for box or batch annealing of silicon steel strip.

Another object of the present invention is to provide a method for box or batch annealing, in which a glassy film is formed on the surface of silicon steel strip.

Other objects of the present invention will be made clear by the following description and attached drawings.

In order to attain the above objects, the present invention comprises a method for annealing silicon steel strip, which is characterized by applying a slurry of such metal-hydroxides as are capable of forming a glassy film by the reaction with SiO contained in the surface of silicon steel strip in the annealing atmosphere, winding the coated steel strip into a coil, annealing said coil in the desired annealing atmosphere to evaporate the combined water of the applied metal-hydroxides, thereby contracting the volume of said metal-hydroxide and producing a space between coil-layers, and annealing with an atmospheric gas flowing uniformly between coil-layers through the thus formed space from the lower end of said coil.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of an apparatus for applying the material for forming a film during the annealing according to the present invention.

FIG. 2 is a sectional view of a batch-type furnace for use in the annealing according to the present invention.

FIG. 3 is a plan view of an embodiment of the base plate of the apparatus for use in the annealing; according to the present invention.

FIG. 4 is a plan view of another embodiment of the base plate of FIG. 3.

FIG. 5A is a sectional view of. the coil-layers before the annealing according to the present invention.

FIG. 5B is a sectionalview of the coil-layers during or after the annealing according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the methodof the present invention, a metalhydroxide which is capable of forming a glassy insulation film by a reaction with SiO contained in the surface of silicon steel strip, is applied to silicon steel strip; after this coating is dried, said steel strip is wound into a coil; the thus formed coil is charged into the annealing furnace and annealed in the desired annealing atmosphere, thereby causing dehydration of the applied metal-hydroxide to evaporate water completely to form a metal-oxide, againthereby contracting the volume of the original metal-hydroxide and producing a space between coil-layers. Thus, the so-called loose coil is formed. The annealing of silicon steel strip carried out while an atmospheric gas is forced into said space, is effective in the formation of a glassy film on the surface of the steel strip and in decarbonization and desulfurization of the steel strip. One example of metal-hydroxides which can be used in the method according to the present invention is magnesium hydroxide, as mentioned below. Of course, the application the hydroxides of alkali earth metals and of other metals is not outside the scope of the present invention.

The application of a slurry of an oxide of alkali earth metals on the surface of silicon steel strip, together with the annealing of the thus treated steel strip to form a glassy film on its surface and concurrently to prevent coil-layers from sticking together during annealing, is alreadypublicly known from the U.S. Pat. No. 2,906,645. So is the method for the improvement of the magnetism of magnetic metal materials, according to which a metal-oxide is applied to such magnetic metal material, thereby generating hydrogen during annealing, such hydrogen reacting with said metal material to improve its magnetism. However, in these methods the effect of volume contraction produced due to evaporation of water from the applied material, and forcing an atmospheric gas into the thus produced space is not utilizedrOn thecontrary, the generated water stays between coil-layers, oxidizing the steel strip and making the product unsatisfactory. Because of these drawbacks, the annealing by the conventional methods requires a longer time.

In order to solve this problem, stepsare taken to reduce the formation of hydrates to a minimum when making a slurry of an oxide of alkali earth metals according'to the conventional methods. According to the present invention, however, the volume contraction of the applied metal-hydroxides during annealing or other treatments is utilized, so that a slurry of said metal-oxide obtained by complete hydration, is applied to the surface of the silicon steel strip so that the volume of the applied metal-hydroxide is reduced on heating, and the thus produced space is utilized to carry out the effective annealing with an atmospheric gas flowing through the space; thus, the method of the present invention solves said problem of the generated water staying between coil-layers and improves the adherence, insulation and uniformness of the glassy film.

An embodiment of the present invention is set forth hereinafter.

As shown in FIG. 1, a slurry of magnesium hydroxide is applied to the silicon steel strip 4 which has been reduced to the final thickness and wound into a coil 3. Magnesium hydroxide can be dehydrated to a volume of 0.54 that of its initial volume; however, such volume reduction is not as great is the magnesium oxide has not been hydrated sufficiently. A comparison of the weight of a mixture of water and MgO after being dried at 1 10 C. to remove free water, with that which has been heated at 1,000 C. to remove combined water, reveals that the content of Mg(OI-l) in the mixture of water and MgO in the former case is reduced to 80 percent in the latter case, the temperature of water of both cases being 30 40 C. when measured. Though it varies according to the baking conditions of the MgO, the most appropriate water temperature for the hydration of MgO is between 30 and 40 C., to which range the water temperature must be adjusted. The most appropriate mixing ratio between water and MgO is about 1 g. MgO for 10 cc. water. A magnesium slurry l with a degree of high hydration preferably prepared as above is held in the container 2; the silicon steel strip 4 uncoiled from the coil 3, runs through said container 2; the volume of the applied material is adjusted by the roll 5. Then, the steel strip 4 is conveyed through the drying room 6, and dried to such an extent that only the free water is removed. After being dried, it is wound in a coil 7. The larger the volume of the applied material, the better; but, the most appropriate volume is in a range of 10 to 30 g./m According to the present invention, the volume of applied slurry of metal-hydroxide having a degree of hydration of 80 percent can be the low amount in the above range; the volume should be increased as the degree of hydration drops.

A film 8 made of magnesium hydroxide which has been applied on the surface of the silicon steel strip 4 and dried, retaining combined water, is interposed between the spires 4 and 4 of the coil 7 of said steel strip, as shown in FIG. A. When the coil of said steel strip 4 is charged into the annealing furnace 10, said magnesium hydroxide is heated by the annealing heat and decomposes and contracts as it give up combined water, thereby producing a space 15 as shown in FIG. 5B. Into said space is forced an atmospheric gas. It is preferred that said annealing furnace 10 be a batch-type furnace in which it is possible to adjust the composition and pressure of an atmospheric gas, as shown in FIG. 2. When the space is produced between coil-spires, as mentioned above, an atmospheric gas flows through said space 15 of the coil 9 from the bottom to the top as indicated by the arrows in the figure, making possible the annealing of the steel strip 4 wound into the coil 9 in a short time. The base plate 11 for supporting the coil in said annealing furnace should be provided with many small holes 12, as illustrated in FIG. 3, so that gas can be forced into the space through them. FIG. 4 is a plan view of another embodiment of the base plate. The base plate of FIG. 4 is provided with an appropriate number of the gas supplying holes 14 and such a number of radial grooves 13, so that the supplied gas can be uniformly forced into the coil of the steel strip 4 placed on the plate from the bottom end of the coil. As an auxiliary step, it is recommended that the upper end of the center hollow of the coil be covered by a plate 16. Due to the supplied atmospheric gas, said magnesium hydroxide 8 decomposes and give up water in the form of steam, producing the space 15, as shown in FIG. 5B. When the volume of applied metal-hydroxides is, say, 18 g./m.", a space of about 15 to p. is produced at a heating temperature between 500 and l,200 C., taking the expansion of the coil-layer into account. When the space 15 has been produced, an atmospheric gas can go through it very swiftly, making possible the annealing of the silicon steel strip 4 in a short time.

EXAMPLE MgO was mixed with water having a volume of three times as much as MgO at 30 C., and a slurry of magnesium hydroxide with a degree of hydration of 80 percent was obtained. This slurry was applied to silicon steel strip of the following specifications:

(a) Chemical composition: C 0.006 7:, Si 3.1 7:, Mn 0.1%, and

residual Fe and unavoidable impurities (b) Width of coil: 900 mm. (c) Thickness of strip: 0.35 mm.

The volume of the slurry of magnesium hydroxide was adjusted by the roll to 20 g./m The thus treated steel strip was dried at 250 C. for several minutes; then it was wound into a coil. Then, said coil was charged into the finishing annealing furnace and annealed at 1,150 C.

As a result, the annealing time was reduced to 30 to 40 percent, compared with the annealing time required by the conventional method. The tests on the thus treated steel strip after the additional treatment with phosphate, show that it has improved film adherence, resistance between coil-layers, magnetism and flexibleness evenly in the direction of width and length.

The thus obtained improvements are:

Layer resistance: Space factor: Magnetism: Flexibleness:

What we claim is:

l. A method of annealing silicon steel strip for decarburizing, finish annealing and the like, which comprises the steps of applying a water slurry of a metal hydroxide in which the metal hydroxide has a large amount of combined water onto the surface of silicon steel strip in an amount of from 10 to 30 gm./m. for forming a glassy film on said strip surface, the metal hydroxide having a degree of hydration of about percent when it is applied in an amount of 10 gm./m. and correspondingly lesser degrees of hydration when applied in greater amounts, drying the slurry applied to the surface of said steel strip to remove substantially all the uncombined water from the slurry, winding the coated silicon steel strip into a coil in which the surface of the individual spires contact each other, placing said coil in an annealing furnace'with the coil axis directed vertically, annealing said coil at an annealing temperature to drive off the combined water of the hydroxide, thereby causing a volume contraction of said hydroxide so as to leave a space between the individual spires of said coil, and during the annealing, forcing atmospheric gas through said spaces from the bottom of said coil.

2. A method as claimed in claim 1 wherein said drying is carried out at about 250 C. for several minutes.

3. A method as claimed in claim 1 wherein the metalhydroxide slurry is a slurry of the hydroxide of an alkali earth metal.

4. A method as claimed in claim 1 wherein the metalhydroxide slurry is a slurry of magnesium hydroxide made by mixing in water at a temperature between 30 and 40 C. at a mixing ratio of one gram MgO per 10 cc. water.

5. A method as claimed in claim 1 wherein said annealing step is carried out at a temperature between 500 and l,200 C.

6. A method as claimed in claim 1 wherein said annealing furnace used in the annealing step to heat said coil is a boxtype furnace.

7. A method as claimed in claim 1 wherein said annealing furnace used in the annealing step to heat said coil is a batchtype furnace which allows adjustment and feed of the atmospheric gas.

8. A method as claimed in claim 7 wherein said coil to be treated in the annealing step is placed on a base plate having a multiplicity of holes and is heated while the atmospheric gas is passed through said holes.

9. A method as claimed in claim 1 in which hydroxide is applied in an amount of 20 gm./m 

2. A method as claimed in claim 1 wherein said drying is carried out at about 250* C. for several minutes.
 3. A method as claimed in claIm 1 wherein the metal-hydroxide slurry is a slurry of the hydroxide of an alkali earth metal.
 4. A method as claimed in claim 1 wherein the metal-hydroxide slurry is a slurry of magnesium hydroxide made by mixing in water at a temperature between 30* and 40* C. at a mixing ratio of one gram MgO per 10 cc. water.
 5. A method as claimed in claim 1 wherein said annealing step is carried out at a temperature between 500* and 1,200* C.
 6. A method as claimed in claim 1 wherein said annealing furnace used in the annealing step to heat said coil is a box-type furnace.
 7. A method as claimed in claim 1 wherein said annealing furnace used in the annealing step to heat said coil is a batch-type furnace which allows adjustment and feed of the atmospheric gas.
 8. A method as claimed in claim 7 wherein said coil to be treated in the annealing step is placed on a base plate having a multiplicity of holes and is heated while the atmospheric gas is passed through said holes.
 9. A method as claimed in claim 1 in which hydroxide is applied in an amount of 20 gm./m2. 